PATTERNS OF MAGNETIC FIELDS
Magnetic field pattern can easily be observed using iron fillings. The magnetic is put on paper and the iron fillings are sprinkled lightly on the paper around the magnet. The paper is tapped gently and the iron fillings are found to turn and set to in definite direction.
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MAGNETIC FIELD AROUND A STRAIGHT CONDUCTOR CARRYING CURRENT
A straight conductor carrying current can be shown that it has magnet filed around it. Allow a thick isolated copper wire to pass vertically through a hole in a card board shit. As shown below, sprinkle some iron fillings uniformly on the cardboard around the vertical wire connect the ends of the wire to a battery, switch on the current and place some compass needles around the wire. Note the direction to which the compass needle point. Switch on the current and note the swing of the needles and how they point.
It will be observed that when current is switch on and the card board is gently tapped, the fillings arrange themselves in a series of concentric circles about the wire as centre. Also as soon as the current is switch on, the needles will swing around and form a circle with the wire as centre. The direction of the filed depends on the direction of flow of the current. Such a direction can always be obtained by applying the Right Hand Grip Rule.
FORCE ON A CURRENT CARRYING CONDUCTOR IN A MAGNETIC FIELD
A conductor carrying an electric current, when placed in the magnetic field experiences a mechanical force. This can be demonstrated by using two metal rails fixed on each side of a powerful horse-shoe magnet. A copper rod is placed across the rays. When the current is passed through this copper rod, it is observed that the copper rod rolls along the rays, toward the right. If by adjusting the rheostat, more current is made to flow through the rod. One will notice that the rod moves faster, thus the force on the rod increases when the current increases.
If the direction of flow of current is reversed by reversing the connections at the battery terminals, the rod will be observed to move towards the left, opposite to the previous direction of motion.
If one turns the magnet such that the magnetic field is parallel to the length of the rod as shown below, it will be observed that the current carrying the rod remains stationary no matter the amount of current hat pass through. There is therefore no force on the rod.